1. Field of the Invention
The present invention relates to optical transmission devices, and more particularly, to an optical transmission device for transmitting an optical WDM (Wavelength Division Multiplex) signal.
2. Description of the Related Art
As a result of an explosively increasing demand for data communications chiefly via the Internet traffic, backbone networks are required to have larger capacity and to cover even longer distances. Also, because of diversification of services that users make use of, there has been a demand for networks which have high reliability and flexibility and yet are economical.
Especially, optical communication networks form the basis of the infrastructure of information communication networks. Thus, optical communication networks are required to provide wider coverage of even more sophisticated services and development thereof is currently rapidly advanced for an information-oriented society. In optical transmission systems, WDM technology is widely used as key technology. WDM is a technique whereby light beams of different wavelengths are multiplexed to allow multiple signals to be simultaneously transmitted over a single optical fiber.
In order for various processes to be performed separately on optical paths in an optical wavelength region, a WDM transmission node carries out OADM (Optical Add Drop Multiplex) control for dropping (Drop) or adding (Add) an optical signal of specified wavelength, without converting optical signals to electrical signals.
To achieve OADM, a wavelength tunable filter capable of variably selecting a desired one of the wavelengths multiplexed in a WDM signal is required. As such wavelength tunable filter, an AOTF (Acousto-Optic Tunable Filter) is widely used.
The AOTF filters out a desired wavelength by inducing a change in refractive index of an optical waveguide through an acousto-optic effect (diffraction of light by a sound wave excited in a substance or on the surface thereof), to rotate the polarization state of light propagated through the optical waveguide and thereby separate/select a spectral component. The AOTF can be tuned in over a wide wavelength range by varying the sound wave frequency (RF: Radio Frequency) applied thereto and thus is considered a potential device useful in configuring OADM.
Meanwhile, optical communication networks are subjected to an optical signal continuity test in order to maintain reliability of optical communication. The optical continuity test is performed to determine whether a specific optical signal reaches a predetermined spot or not, and an optical loopback test, for example, is conducted for the purpose. In the optical loopback test, an optical signal is sent out and redirected back at a predetermined spot, to determine whether the redirected signal can be received or not, thereby checking the continuity of a path up to the redirected spot. Such a continuity test permits a faulty device or a faulty spot in a communication line to be located from a remote place, making it possible to enhance the efficiency of maintenance and inspection.
As control techniques for conventional optical continuity tests, there has been proposed a technique wherein a 1×2 optical switch with one input and two branch outputs is provided in an optical communication path and a continuity test is performed with the optical switch switched to a loopback path (see Unexamined Japanese Patent Publication No. H09-18421 (paragraph nos. [0024] to [0026], FIG. 4), for example).
In conventional wavelength selection using an AOTF, wavelength is scanned with an RF signal based on wavelength channel information sent from a node, to count the number of peaks so that a desired wavelength may be tuned in for filtering. This procedure is followed because the temperature dependency of the wavelength selection by an AOTF is as large as about 0.7 nm per 1° C. and the AOTF does not have the function of detecting an absolute wavelength (a fluctuation-free, fixed wavelength on the wavelength axis).
Thus, in the conventional wavelength selection by means of an AOTF, the number of wavelength peaks is counted to detect a desired wavelength, and accordingly, there is a possibility that noise such as a side-peak is erroneously detected as a wavelength signal. If a side-peak is regarded as a substantial peak, then it is not possible to connect with a target node, giving rise to a problem that connection is established with a wrong node different from the target node, for example.
As regards absolute wavelength measurement, a device capable of detecting a desired absolute wavelength has been put on the market (e.g., WDM monitor (WD200) from Yokogawa Electric Corporation), and such a device may be used to carry out wavelength selection. However, the device uses the combination of a diffraction grating for wavelength dispersion and a PD array and thus is costly (over ¥1,000,000 per unit) as well as large-sized (170×220×28 mm). Accordingly, the device is not applicable to nodes which need to be reduced in cost and size, such as those used in a metropolitan area network or an access network.
According to the conventional optical continuity testing technique (Unexamined Japanese Patent Publication No. H09-18421), on the other hand, when a loopback test is performed, the communication path is cut off because of the switching of the optical switch, and therefore, it is not possible to conduct the continuity test on a specific wavelength only.
For example, suppose the case where only a wavelength λ1, among wavelengths λ1 to λ8 multiplexed in a WDM signal, is to be subjected to the continuity test. With the conventional technique, the communication line itself is switched by the optical switch, so that all the wavelengths λ1 to λ8 are redirected back, giving rise to a problem that the communication is disrupted if any node is communicating using the wavelengths λ2 to λ8.
For an OADM node, on the other hand, the capability to add and drop (Add/Drop) a desired wavelength is essential for flexible operation of networks. During operation of a network in which desired wavelengths are selected and separated, if a wrong wavelength is added, data is transmitted to a node different from the target node, possibly causing the node receiving the erroneously transmitted data to go down. Especially, in the case of in-service installation of an extension unit or the like, if an expansion slot is fitted with a wrong unit with different wavelength settings, the operation of the network becomes anomalous, which is fatal to the network.
Conventional OADM networks do not have a wavelength monitoring mechanism and thus are unable to automatically determine whether or not desired wavelengths have been added/dropped. Also, even if an extension unit with different wavelength settings is erroneously inserted at the time of in-service installation of the unit, no one may possibly notice that the unit is communicating with a node different from the settings, because of lack of a wavelength monitoring mechanism. Further, the conventional OADM networks do not have a protective mechanism (fail-safe mechanism) for providing protection in case a wrong unit is inserted, and thus the network goes down if the worst comes to the worst.